The use of simple electrodes to sense bio-potentials has been going on for almost a century. Most of the early techniques relied on sharpened wires or glass micropipette electrodes. Sharpened wire electrodes are still widely used for neural recordings. They are fairly inexpensive and are simple to construct, yet are capable of recording multiple high quality action potentials from localized areas of tissue.
Although wire electrodes are extremely well-suited for neural recording, unless special equipment is used, the recording site is generally limited to the tip of the electrode. Metal electrodes tend to be hand-made, one-at-a-time, making it difficult to precisely control the surface roughness and amount of exposed conductor. The characteristic impedance, a good indicator of future electrode performance, is highly dependent on the state of the exposed site and can drift unpredictably with irregular surface characteristics.
Glass capillaries, drawn to a few microns and filled with an electrolyte, are commonly referred to as glass microelectrodes. Because the glass can be drawn to such small dimensions, they have generally been used as intracellular electrodes, measuring the potentials within (large) individual cells. These electrodes are also easily adaptable to measuring extracellular potentials.
Similarly to wire electrodes, glass microelectrodes are typically hand-made, one-at-a-time. Unlike the cone electrode where the recording electrode is inside the tube with active neurites, the recording site of a typical glass microelectrode is generally at the tip of the electrode, where the internal electrolyte contacts the outside fluid of the electrode. The opening typically varies from electrode to electrode, and the tip tends to get clogged, making the electrode less sensitive.
Near the beginning of the age of the semiconductor integrated circuit, researchers started using batch fabrication techniques and tools developed in silicon foundries to micromachine a variety of bioprobes out of a wide variety of materials. While non-lithographically produced electrodes are ideal for many applications, BIO-MEMS neural interfaces produced by batch fabrication methods common to the semiconductor industry have several advantages, including low cost once mass-produced, high yield, high channel counts, accurately controlled dimensions (for site spacing and geometry) and little variability from electrode to electrode, batch to batch.
The availability of a wide variety of processes and tools specialized for the fabrication of silicon integrated circuitry made silicon a good choice for some of the earliest bio-probes. A variety of silicon-containing composite substrates such as SOI (silicon-on-insulator), silicon nitride with silicon, and polysilicons, have also been tried to varying degrees of success. Ceramic substrates have the advantages of high insulation resistance and no need for expensive cleanroom equipment. Some rugged substrate materials such as molybdenum were tried, but typically were not suitable for chronic usage because of insulation coating problems.
Studies with microelectrode electrical recordings alone can only provide information about local potentials and possible source distributions within biological systems. Therefore, to begin to fully understand complex behavior, it is desirable to be able to deliver precise amounts of chemicals to localized areas while monitoring the system's electrical responses in vivo.
A significant amount of research has been conducted on the micromachining of hypodermic microneedles designed for drug delivery. The advantages of micromachining these needles include small size, reproducibility, low unit cost, and the potential integration of sensors, pumps, and other micro-accessories. Microneedles have been batch-fabricated from a variety of materials, including glass, silicon-silicon nitride, polysilicon, polymers and metal. However, most of these electrodes are meant only for drug delivery and will not simultaneously record action potentials.
The promise of advanced microelectrode systems to significantly improve the quality of life for the impaired hinges on the development of an efficacious and safe implant to interface with the tissue over extended periods. Despite the wide range of materials available and the increasing complexity of the design process, there still exist a number of fundamental problems associated with drug delivery microelectrodes and microelectrode arrays. The largest failure modes for chronically implanted electrodes are thought to be the inflammation of tissue due to the microelectrode, and micromotion-induced trauma. While batch-fabricated BIO-MEMS electrodes offer many advantages over wire electrodes due to the wide choice of biocompatible materials, reliable interconnects and sophisticated processing schemes, the failure modes of the electrodes largely remain the same. Accordingly, there is a need for a reliable, consistent, and long-term microscale system for interfacing directly with biological systems which overcomes the shortcomings of previously developed systems.